The invention relates to sensors, and more particularly, to a sensor that measures air pressure.
Air pressure sensors have found general acceptance as a reliable way to measure the relative altitude of an object. For instance, the altitude of an aircraft may be determined based on readings from an air pressure sensor.
Known pressure sensors use a sensor die, typically in integrated circuit form, to measure the difference between a reference pressure and a pressure that is applied to or sensed by the sensor die. The sensor die is mounted within a sensing assembly, which may be a housing or capsule, to protect the sensor die from damage. A portion of the capsule is evacuated to provide a reference pressure. Sensed air pressure is ported or otherwise connected to the interior of the capsule so that the sensor die can compare the sensed air pressure to the reference pressure. In another sensor design, a portion of the capsule interior is sealed off to prevent corrosion of the circuitry due to atmospheric conditions.
One problem with known air pressure sensors is that a sensor die becomes subject to stress when the sensor die is attached to the capsule. This stress may compromise the delicate pressure measurements that must be made by the sensor.
Another problem is that high-humidity environments cause the sensor to be subject to some inaccuracy. It is postulated that high humidity causes an oxide layer to form and thicken on the surface of the sensor die. This adversely affects pressure readings over a period of time.
Still another problem is that the manufacturing processes necessary to create a vacuum reference pressure within the sensor assembly are time-consuming and expensive. Furthermore, while such processes have been found satisfactory for some sensor applications, other applications, such as vertical height measurements during the flight of an aircraft, require a higher degree of accuracy than may be economically provided by a sensor that is manufactured using such processes.
It is therefore an object of the invention to provide a pressure sensor that is accurate enough to be used in aerospace applications.
It is another object of the invention to provide a pressure sensor that is relatively unaffected by high humidity environments.
It is still another object of the invention to provide a pressure sensor that is manufactured in a way so as to minimize the effects of the manufacturing processes on the measurements made by the pressure sensor.
A feature of the invention is the use of manufacturing techniques that eliminate sources of measurement error.
An advantage of the invention is that a reliable and extremely accurate yet cost-effective pressure sensor may be produced.
The invention provides a method of manufacturing an air pressure sensor. A plurality of sensor circuitry modules are formed on a first side of a first wafer. A plurality of cavities on a second side of the first wafer are formed. Each of the plurality of cavities is aligned with one of the plurality of sensor circuitry modules. A second wafer is attached to the second side of the first wafer. The first wafer and the second wafer are cut such that a single sensor circuitry module is separated from the other sensor circuitry modules, wherein a portion of the second wafer remains attached to the sensor circuitry module so that the cavity opposite the sensor circuitry module is enclosed by the portion of the second wafer to form a reference pressure chamber therebetween. The sensor circuitry module is flexibly mounted to an interior surface of a sensor housing. The sensor circuitry module is electrically connected to external conductive connectors.
The invention also provides an air pressure sensor, which includes a sensor die that is exposed on at least one side to an air pressure that is to be measured. A glass portion is secured to the sensor die such that a vacuum reference cavity is enclosed therebetween. A housing includes an enclosure in which the sensor die and glass portion are housed, the enclosure being pneumatically connected to the air pressure that is to be measured. An electrical connector communicates air pressure data from the sensor die. A plurality of unconnected adhesive elements flexibly attach the glass plate within the enclosure.
The invention further provides a method of manufacturing an air pressure sensor. A sensor die is connected to a backing plate such that a cavity of reduced pressure is enclosed therebetween. The backing plate is flexibly attached to an interior surface of a housing using an adhesive. The adhesive is applied such that the backing plate is connected to the interior surface of the housing at a plurality of separate, discrete areas.